Hydraulic system and automatic gearbox

ABSTRACT

A hydraulic system including a multi-flow hydraulic pressure supply unit, especially a dual-flow hydraulic pressure supply unit, such as a pump, by which a volumetric flow of pressurized fluid is supplied to a hydraulic-fluid-operated device. A valve arrangement is provided either for switching between the individual pump flows or for interconnecting the individual pump flows.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 10/510,140, filedon May 23, 2005, which is a continuation of International ApplicationSerial No. PCT/DE03/01194, with an international filing date of Apr. 10,2003, and designating the United States, the entire contents of which ishereby incorporated by reference to the same extent as if fullyrewritten.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a hydraulic system with a multi-flowhydraulic pressure supply unit, especially a dual-flow hydraulicpressure supply unit, such as a pump, through which a volumetric flow ofhydraulic fluid is fed to a hydraulic-fluid-operated device. Theinvention also relates to an automatic transmission for motor vehicles.

2. Description of the Related Art

In modern motor vehicles, hydraulic systems in which at least onepressure supply unit supplies at least one hydraulic-fluid-operateddevice with a defined pressure are being used to improve safety andcomfort. The known hydraulic systems indeed have a high power density, alow power-weight ratio, and high dynamics, but they nonetheless requiremore energy in relation to regulated electric drives, which leads tohigher fuel consumption. Previously, mostly single-flow pumps have beenused to supply pressure to, for example, automatic transmissions.Single-flow here means that the pump conveys one pump flow. In contrast,multi-flow pumps convey several pump flows independently of one another.The pump flows are thus connected in parallel.

An object of the present invention is to reduce the losses in knownhydraulic systems. The hydraulic pressure supply unit used in thehydraulic system should meet the demands of the automotive industry overa wide range of hydraulic requirements.

SUMMARY OF THE INVENTION

The object is achieved by a hydraulic system with a multi-flow hydraulicpressure supply unit, especially a dual-flow, hydraulic pressure supplyunit, such as a pump, through which a volumetric flow is fed to ahydraulic-fluid-operated device. A valve for switching between theindividual pump flows and/or for interconnecting the individual pumpflows is provided. The valve enables actuating the individual pump flowsselectively. It is possible to actuate only one or several pump flowswith a single valve, as needed.

A preferred embodiment of the hydraulic system is characterized in thatthe individual pump flows are joined or separated through a check valve.The check valve makes it possible to conduct away at least one of thepump flows so that optionally only at least one of the pump flows isconveyed to the hydraulic-fluid-operated device.

A further preferred embodiment of the hydraulic system is characterizedin that the at least one pump flow, which is separated by the checkvalve from the at least one other pump flow, can be conducted awaythrough the valve. Only one pump flow or several pump flows are suppliedto the hydraulic-fluid-operated device as a function of the setting ofthe valve. The check valve prevents all the pump flows from beingconducted away.

A further preferred embodiment of the hydraulic system is characterizedin that the valve includes an end surface that is biased by a spring,which surface is acted upon with the dynamic feedback pressure leadingfrom the hydraulic-fluid-operated device to the input side of thehydraulic pressure supply unit. The use of the dynamic feedback pressurefor actuating the valve assures that the valve switches from asingle-flow to an at least dual-flow conveyance of the hydraulicpressure supply unit when the dynamic feedback pressure drops below aspecified minimum value. If the dynamic feedback pressure exceeds aspecified maximum value, the valve switches from an at least dual-flowto an at least single-flow delivery from the hydraulic pressure supplyunit.

A further preferred embodiment of the hydraulic system is characterizedin that a hydraulic resistance is arranged between the valve and theinput side of the hydraulic pressure supply unit. The hydraulicresistance serves to generate the dynamic feedback pressure foractuating the valve.

A further preferred embodiment of the hydraulic system is characterizedin that the valve includes a 2/2 way valve that releases a connectionprovided between the output side of a pump flow and the input side ofthe hydraulic pressure supply unit in the one position, the connectionbeing interrupted in the other position of the 2/2 way valve. The 2/2way valve makes it possible to feed the two pump flows to thehydraulic-fluid-operated device individually or together, depending uponneed.

A further preferred embodiment of the hydraulic system is characterizedin that the valve has three shifting stages, whereby in the firstshifting stage a cooling circuit is not supplied and only one pump flowis conveyed to the hydraulic-fluid-operated device by the hydraulicpressure supply unit. In the second shifting stage the cooling circuitis not supplied and at least two pump flows are conveyed to thehydraulic-fluid-operated device from the hydraulic pressure supply unit.In the third shifting stage the cooling circuit is supplied and at leasttwo pump flows are conveyed to the hydraulic-fluid-operated device fromthe hydraulic pressure supply unit. The shifting stages make it possibleto use the valve, which can be actuated as needed, for also turning thecooling system on and off, for example of a clutch.

A further preferred embodiment of the hydraulic system is characterizedin that the valve has a further shifting stage in which the coolingcircuit is not supplied and a safety valve is operated. The safety valvecan serve, for example, to prevent overheating of the medium conveyed,or can represent a redundant opening mechanism for a clutch.

A further preferred embodiment of the hydraulic system is characterizedin that the valve, especially as a 2/2 way valve, is designed such thatonly one pump flow is conveyed from the hydraulic pressure supply unitto the hydraulic-fluid-operated device as long as a first pressure,especially the adjustment pressure of an automatic transmission, issmaller than or equal to the sum of a second pressure, especially thecontact pressure of an automatic transmission, and of a spring force,and in that at least two pump flows are conveyed from the hydraulicpressure supply unit to the hydraulic-fluid-operated device if the firstpressure, especially the adjustment pressure of an automatictransmission, is greater than the sum of the second pressure, especiallythe contact pressure of an automatic transmission, and of the springforce. In that way, it is assured that connection of at least onefurther pump flow will take place as a function of need.

A further preferred embodiment of the hydraulic system is characterizedin that the valve includes a spool whose one end face is acted upon bythe first pressure and whose other end face is acted upon by the secondpressure and the spring force. The spool moves as a function of theforces acting upon it and thus releases a flow path for one or more pumpflows.

A further preferred embodiment of the hydraulic system is characterizedin that the valve performs even additional valve functions in additionto turning on or shutting off the pump flow (the first valve function)in that the valve spool releases or closes openings on other controlunits. Those additional valve functions can, for example, be anapplication of pressure on a hydraulic clutch, or the application ofpressure on the conical pulleys of a continuously variable transmission.The coupling of the first valve function with a further valve functionrepresents a cost advantage because, instead of two slides and twoboreholes, only one need be manufactured or machined.

The coupling of the first valve function with a further valve functionlikewise represents a functional advantage if a volumetric flowrequirement is controlled through the further valve function whosecoverage takes place in the same valve by turning on a pump flow. Thecoupling of the first valve function with a further valve functionlikewise represents a functional advantage when those valve functionscan take place in connection with different motions of the spool, thatis, in part independently. For example, the additional valve functioncan represent the application of pressure on a clutch for the secondgear, and the spool now permits connecting an additional pump flow by afurther motion of the spool when the clutch is not actuated. Thatcontrol and especially further displacement of the spool takes place ina known manner, for example by applying a small electronicallycontrolled pressure to an end face of the spool.

A further preferred embodiment of the hydraulic system is characterizedin that the valve includes at least two valves whose switchingrespectively brings about the conveyance of at least one of the pumpflows to the hydraulic-fluid-operated device. Both valves in each caseassume even further functions, as described above, for examplerespectively subjecting a clutch to pressure.

A further preferred embodiment of the hydraulic system is characterizedin that the two valves are connected in series. Switching from one ofthe two valves leads to at least one of the pump flows being conveyed tothe hydraulic-fluid-operated device. The at least one pump flow isconveyed back to the input side of the hydraulic pressure supply unit.

A further preferred embodiment of the hydraulic system is characterizedin that a volumetric flow regulating valve is arranged between theoutput side of the pressure supply unit and the hydraulic-fluid-operateddevice. The volumetric flow regulating valve serves to regulate thevolumetric flow to the hydraulic-fluid-operated device. The excessvolumetric flow is conveyed back to the input side of the hydraulicpressure supply unit.

A further preferred embodiment of the hydraulic system is characterizedin that connection/disconnection of individual pump flows takes place asneeded. For example, fixed rotational speed thresholds can be defined,at which the switching takes place. Here care should be taken that therequisite need is still being covered when disconnecting a pump flow.

A further preferred embodiment of the hydraulic system is characterizedin that the ratio between the individual pump flows is asymmetrical.Thereby it is possible to convey three different volumetric flows withone hydraulic pressure supply unit.

A further preferred embodiment of the hydraulic system is characterizedin that a first pump flow covers approximately a third and a second pumpflow approximately two thirds of the overall conveyed flow of thehydraulic pressure supply unit. With a corresponding control logic, bothpump flows together can supply 100% or only one pump flow can supply 33%or 66% of the entire conveyed flow of the hydraulic supply unit, asneeded.

A further preferred embodiment of the hydraulic system is characterizedin that the hydraulic pressure supply unit includes a vane pump or aninternal gear pump.

A further preferred embodiment of the hydraulic system is characterizedin that a hydraulic resistance is arranged between the valve and theinput of the hydraulic pressure supply unit. An injector pump isincorporated in the vane pump. The injector pump is used to assureproper filling of the preferably mechanically driven pump at higherrotational speeds.

The above-identified object is achieved in connection with an automatictransmission for motor vehicles through a previously described hydraulicsystem. The hydraulic system in accordance with the invention can also,however, be used in the steering system or the anti-roll system.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages, features and details of the invention becomeapparent through the description below, in which various embodiments aredescribed in detail with reference to the drawings. There is shown:

FIG. 1 a hydraulic circuit diagram of a hydraulic system in accordancewith an embodiment of the invention for controlling an automatictransmission;

FIG. 2 a block diagram of an arrangement for controlling a dual-flowpump with a valve and an additional flow regulating valve;

FIG. 3 a block diagram of an arrangement for controlling a dual-flowpump with switching dependent upon two different pressures;

FIG. 4 a block diagram of an arrangement for controlling a dual-flowpump with a volumetric flow; and

FIG. 5 a block diagram of an arrangement for controlling a dual-flowpump with two valves connected in series.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Contemporary automatic transmissions for passenger cars control thestarting process, the transmission ratio change, and the operation ofthe reversing clutch for forward/reverse travel, as well as for coolingand lubricating hydraulically. A hydraulic pressure supply unit and ahydraulic control unit are necessary for those purposes.

Up until now, largely single-flow pumps have been used for supplyingpressure to automatic transmissions. The pumps are distinguished in thattheir conveyed volumes are directly proportional to their rotationalspeed. That is disadvantageous insofar as, in designing pump sizes,often extreme situations such as, for example, a rapid adjustment at lowrotational speeds, are design specific. In many other drivingsituations, the volumetric flow then made available is not necessary.The efficiency of those pumps is not optimal since, for example,unnecessarily high hydraulic power is generated by the mechanicallydriven pump at maximum speed. Second, the hydraulic components used,such as, for example, the pump, are subjected to unnecessarily highstress at maximum pump speed.

A pump concept is provided by the present invention, in which therequired volumetric flow is generated as a function of demand. In thatway, the dissipated hydraulic power, as well as the stress on the pump,can be reduced. With the hydraulic system in accordance with the presentinvention, it proves to be advantageous that the system pressure can bereduced, since at a high system pressure leakages as a rule increase.The control of the individual pump flows is preferably carried out insuch a way that the net volumetric flow remains unchanged. Generatingless volumetric flow at the same rotational speed with cold oil thanwith warm oil is desirable, since at low oil temperatures less leakageis present and consequently the volumetric flow requirement drops.

It has proven to be advantageous if a portion of the volumetric flowmade available by the pump is also used for cooling the transmission.With cold outside temperatures, it is advantageous to reduce thevolumetric flow to the point that only the precisely required amountflows through the radiator, and consequently heat losses are reduced.With a very hot transmission, it is advantageous to generate morevolumetric flow than needed in order to increase heat transfer.

It has likewise proven to be advantageous if a portion of the volumetricflow furnished by the pump is also used for cooling individualcomponents of the transmission in danger of overheating, for examplefriction clutches. At low friction load it is advantageous to shut thevolumetric flow off, while at high friction load (starting on amountain), the required amount of cold oil is directed over the frictionclutch to protect the latter from overheating.

A hydraulic control unit for an automatic transmission with an inputdisk set 1 and an output disk set 2 is shown in FIG. 1. The hydraulicsystem shown in FIG. 1 also serves to control a clutch 4 for reversetravel and a clutch 5 for forward travel. Actuation of the disk sets 1,2 and the clutches 4, 5 takes place through a pump 8 in which a firstpump flow 9 and a second pump flow 10 are generated parallel to eachother. The two pump flows 9 and 10 are brought together through a checkvalve 12. An additional valve 14 serves to switch between the two pumpflows 9, 10, such that either only pump flow 9 or both the pump flows 9,10 are conveyed together in the direction of hydraulic-fluid-operateddevices 1, 2, 4, and 5.

A preferred embodiment for selective control of two pump flows is shownin FIG. 2. One pump 18, a vane pump, for example, is constructed anddesigned such that a first pump flow 19 is conveyed parallel to a secondpump flow 20. The two pump flows 19 and 20 are connected with each otheron the output side of the pump 18 through a conduit 22 in which a checkvalve 23 is arranged. The check valve 23 is arranged in the conduit 22such that either only the first pump flow 19 or the first pump flow 19as well as the second pump flow 20 are conveyed through a conduit 24 toa hydraulic-fluid-operated device 25. An orifice plate 26 is arranged inthe conduit 24 between the output side of the pump 18 and thehydraulic-fluid-operated device 25. The orifice plate 26, as isindicated through a dotted arrow 28, is part of a flow regulating valve29, which is arranged between the input side and the output side of thehydraulic-fluid-operated device 25 in order to regulate the volumetricflow that is supplied to the hydraulic-fluid-operated device 25. A 2/2way proportional way valve is installed as the flow regulating valve 29.If the volumetric flow conveyed to the hydraulic-fluid-operated device25 exceeds an adjustable maximum value, then the flow-regulating valveswitches into its second position from the position shown in FIG. 2. Inits second position (not shown), the flow-regulating valve 29 releases aconnection from the output side of the pump 18 through the conduit 24past the hydraulic-fluid-operated device to a return conduit 30 thatleads to a tank 31.

An arrow 32 indicates that the pressure in the return conduit 30 servesto control an additional valve 34. The valve 34 is a 2/2 wayproportional valve that interrupts or releases (not shown) a connectingconduit 35 between the return conduit 30 and the output side of thesecond pump flow 20 of the pump 18. The pressure in the return conduit30 is restricted upstream of the valve 34 by a hydraulic resistance 36.

With the hydraulic system shown in FIG. 2, the valve 34 is used toconvey selectively either only pump flow 19 or the two pump flows 19 and20 together to the hydraulic-fluid-operated device 25. The back pressurein the conduit 30 is used, as indicated by the arrow 32, to shift thevalve 34 against a spring. The hydraulic fluid volumetric flow flowingback over the return conduit 30 into the tank 31, and/or toward theinput side of the pump 18, meets the hydraulic resistance 36, whichgenerates the back pressure as a function of the volumetric flow. Theinjector pump used in vane pumps can be used as a hydraulic resistance36, for example. Such an injector pump is needed in vane pumps to assureproper filling of the pump at higher rotational speeds. The hydraulicresistance 36 is shown as an orifice plate in FIG. 2.

The dual-flow pump 18 is driven by a crankshaft of an internalcombustion engine, for example, and consequently conveys a hydraulicfluid volumetric stream as a function of pump rotational speed. The twopump flows 19 and 20 are brought together by the check valve 23 and aresupplied to the hydraulic-fluid-operated device 25 through the conduit24 and the orifice plate 26 when the control valve 34 is in the positionshown in FIG. 2. As for the hydraulic-fluid-operated device, it can be,as shown in FIG. 1, a disk set for adjusting the transmission ratiocondition or a clutch in an automatic transmission. Hydraulic fluidflowing back from the hydraulic-fluid-operated device 25 through thereturn conduit 30 is supplied to the pump 18 again through the hydraulicresistance 36.

If the rotational speed of the pump 18 is increased, then more hydraulicfluid is needed and is fed to the hydraulic-fluid-operated device 25.The return flow of hydraulic fluid through the return conduit 30 to thesuction section of the pump 18 is correspondingly higher. The backpressure increase at the hydraulic resistance 36 owing to that leads tothe valve apparatus 34 shortcircuiting the second pump flow 20 of thepump 18 with the return flow of hydraulic fluid in the return conduit30. In that way, the pressure of the second pump flow 20 decreases tothe back pressure in the return conduit 30 and the check valve 23closes. The second pump flow 20 of the pump 18 is then switched torecycling and needs only to convey against the back pressure in thereturn conduit 30, which would be present in any case. In that way, thepower required by the pump 18 is reduced.

If the rotational speed of the pump 18 increases, and therewith of thevolumetric flow fed to the hydraulic-fluid-operated device 25, then thevolumetric flow regulating valve 29 connected with the orifice plate 26restricts the volumetric flow to the hydraulic-fluid-operated device 25.The hydraulic system shown in FIG. 2 affords the advantage that thesecond pump flow 20 is only added when needed. If thehydraulic-fluid-operated device 25 needs more hydraulic fluid, andconsequently no or little hydraulic fluid is being passed back throughthe flow regulating valve 29 and the return conduit 30 to the tank 31 ortoward the input side of the pump 18, then the volumetric flow decreasesdue to the hydraulic resistance 36, and therewith the back pressure inthe return conduit 30. That causes the valve 34 to close, and thehydraulic fluid volumetric flow of the second pump flow 20 to beconducted back to the hydraulic-fluid-operated device 25, as shown inFIG. 2. That ensures the increased need for hydraulic fluid. If the needof the hydraulic-fluid-operated device 25 decreases again, thencorrespondingly more hydraulic oil is fed back, which leads to a renewedswitching of the second pump flow 20 to pressureless recycling.

A portion of a hydraulic block diagram is shown in FIG. 3 in which avalve for need-dependent volumetric flow generation is designated by 37.The valve 37 includes a valve housing 38 that is connected with theoutput side of a pump having two pump flows 41 and 42, through a conduit39. The two pump flows 41 and 42 are connected with each other through aconduit 45 in which a check valve 46 is arranged. The fact that theoutput side of the pump flows 41 and 42 is connected with ahydraulic-fluid-operated device (not shown) is indicated by an arrow 47.The input side of the pump flows 41 and 42 is connected with a hydraulictank 49. A conduit 50 leads from the hydraulic tank 49 to the valvehousing 38. A spool 52 is biased by a spring 53 in the valve housing 38.The end face of the spool 52 facing away from the spring 53 is actedupon by a pressure P₁. The end face of the spool 52 facing away from thespring 53 is acted upon by a pressure P₂.

In the condition of the valve 37 shown in FIG. 3, the pump flow 42 isrecirculated through the conduit 50 and does not reach thehydraulic-fluid-operated device. Only the pump flow 41 reaches thehydraulic-fluid-operated device through conduit 47. It is assured by thecheck valve 46 that pump flow 41 does not reach the valve housing 38through conduit 39. When the spool 52 moves such that the conduit 39 isclosed, the pressure at the output of the pump flow 42 rises until thecheck valve 46 in the conduit 45 opens the connection to the pump flow41. Then both pump flows 41 and 42 are conveyed to thehydraulic-fluid-operated device. Connection of the pump flow 42 to pumpflow 41 takes place when the product of a first constant and adjustingpressure P₂ is greater than the product of a second constant and the sumof the contact pressure P₁ and the force of the spring. The pressures P₁and/or P₂ can alternatively be a pressure that controls a functionrequiring volumetric flow already present in the hydraulic control unit,or a pressure produced by an electric control apparatus controlled by apilot valve.

The circuit diagram of a hydraulic system is shown in FIG. 4 in which afirst pump flow 56 as well as a second pump flow 57 of a vane pump arefed from a tank 55. The two pump flows 56 and 57 are connected with eachother through a check valve 58. The output of the first pump flow 56 isconnected with a valve housing 62 of a valve 63 through a conduit 60.The second pump flow 57 is connected with the valve housing 62 through aconduit 61. A spool 64 is moveably received in the valve housing 62 ofthe valve 63. The spool can carry out even further functions in the leftregion (not shown), in that oil channels can be closed or openedaccording to the position of the spool. A connection to ahydraulic-fluid-operated device (not shown) is indicated by an arrow 65that proceeds from the valve housing 62. Moreover a return conduit 66proceeds from the valve housing 62 and opens into the tank 55.

In the position of the spool 64 as shown in FIG. 4, the first pump flow56 is conveyed over the conduits 60 and 65 to thehydraulic-fluid-operated device. The second pump flow 57 is conveyedback into the tank 55 through conduit 61 and the return conduit 66. Whenthe spool 64 is moved to the right, the connection between the conduit61 and the return conduit 66 is interrupted, which leads to the pressurerising on the output side of the second pump flow 57 until the checkvalve 58 opens and the two pump flows 56 and 57 are conveyed togetherover conduits 60 and 65 to the hydraulic-fluid-operated device.

A circuit diagram of a hydraulic system is shown in FIG. 5 in which afirst pump flow 71 as well as a second pump flow 72 are fed withhydraulic fluid from a tank 68. The two pump flows 71 and 72 areconnected with each other through a check valve 73. A conduit tohydraulic-fluid-operated devices (not shown) is indicated by an arrow74. A connection between the output side of the second pump flow 72 to avalve housing 76 of a first valve is indicated by an arrow 75. A spool77 is moveably received in the valve housing 76. The valve housing 76 ofthe first valve is connected with a valve housing 79 of the second valvethrough a conduit 78. A spool 80 is moveably received in the valvehousing 79 of the second valve. The valve housing 79 of the second valveis connected with the tank 68 through a return conduit 81.

In the left region not shown, each of the valves can carry out evenfurther functions with the same spool in that oil channels are closed oropened. For example, clutches can be pressurized, or particularhydraulic-fluid-operated devices requiring volumetric flow, such as acooling system, can be switched on.

The second pump flow 72 is conveyed through the conduits 75, 78, and 81back into the tank 68, thus recirculated, in the positions of the spools77 and 80 in the associated valve housings 76 and 79 as shown in FIG. 5.The connection between the two pump flows 71 and 72 is interrupted bythe check valve 73. If one of the spools 77 and 80 is moved to the rightagainst the associated stop, then the return flow into the tank 68 isinterrupted. That leads to a rise in pressure on the output side of thesecond pump flow 72. The pressure on the output side of the second pumpflow 72 rises until the check valve 73 opens, and both pump flows 71 and72 are jointly conveyed through the conduit 74 to thehydraulic-fluid-operated device. The two valves with valve housings 76and 79 are thus switched in a row or in series.

The claims submitted with the application are formulation proposalswithout prejudice to obtaining further-reaching patent protection. Theapplicant reserves the right to claim additional feature combinationspreviously disclosed only in the description or the drawings.

References in the dependent claims refer back to the further developmentof the object of the main claim by the features of the respectivedependent claim. They are not to be understood as a waiver of attainingan independent, objective protection for the feature combinations of thereferred back dependent claims.

Since the objects of the dependent claims can form their own andindependent inventions with respect to the state of the art on thepriority day, the applicant reserves the right to make them the objectof independent claims or divisional applications. They can furthermorealso contain independent inventions that have an independentconfiguration from the objects of the preceding dependent claims.

The embodiments are not to be understood as a restriction of theinvention. Rather, numerous changes and modifications are possible inthe framework of the present invention, especially such variants,elements, and combinations and/or materials, that can be inferred, forexample, by combination or modification of individual features incombination with the general description and embodiments, as well asfeatures described in the claims and contained in the drawings, orelements and procedural steps which can be inferred by the specialistwith respect to accomplishing the objective and that lead by combinablefeatures to a new object, or to new procedural steps, or to newprocedural step sequences, also to the extent that they concernmanufacturing, testing, and operating procedures.

1. A hydraulic system with a dual-flow hydraulic pressure supply unitfrom which a volumetric flow of hydraulic fluid is fed, said hydraulicsystem comprising: a unitary hydraulic pressure supply unit forproviding from a first fluid outlet a first hydraulic fluid output flowand from a second fluid outlet a second hydraulic fluid output flow; ahydraulic-fluid-operated device operatively connected with the pressuresupply unit for receiving hydraulic fluid from the pressure supply unit,wherein the hydraulic-fluid-operated device is a continuously variabletransmission; a check valve positioned between and connected with eachof the first and second fluid outlets for selectively allowing andblocking flow from one of the fluid outlets to thehydraulic-fluid-operated device; and a flow regulator for selectivelyswitching between the first hydraulic fluid outlet flow and the combinedfirst and second hydraulic fluid output flows with thehydraulic-fluid-operated device, wherein at least one fluid outlet flowis separated by the check valve from the other fluid outlet flow, andwherein the system includes a return conduit for conducting at least onefluid outlet flow away from the hydraulic-fluid-operated device throughthe flow regulator, and wherein the flow regulator includes a firstvalve having a first surface biased by a spring and a second surfacethat is acted upon by a back pressure in the return conduit.
 2. Ahydraulic system according to claim 1, wherein the flow regulatorincludes three shifting stages whereby in a first shifting stage acooling circuit is not supplied with hydraulic fluid and only one pumpflow is conveyed from the hydraulic pressure supply unit to thehydraulic-fluid-operated device, whereby in a second shifting stage thecooling circuit is not supplied with hydraulic fluid and at least twopump flows are conveyed from the hydraulic pressure supply unit to thehydraulic-fluid-operated device, and whereby in a third shifting stagethe cooling circuit is supplied with hydraulic fluid and at least twopump flows are conveyed from the hydraulic pressure supply unit to thehydraulic-fluid-operated device.
 3. A hydraulic system according toclaim 2, wherein the flow regulator includes a further shifting stage inwhich a safety valve is activated.
 4. A hydraulic system according toclaim 1, wherein the flow regulator is a 2/2 way valve that allows onlyone pump flow to be conveyed from the hydraulic pressure supply unit tothe transmission as long as a first pressure for adjusting thetransmission is smaller than or equal to the sum of a second, contactpressure applied to components of the transmission and a valve springforce, and wherein at least two pump flows are conveyed from thehydraulic pressure supply unit to the transmission when the firstpressure for adjusting the transmission is greater than the sum of thesecond, contact pressure applied to the components of the transmissionand the spring force.
 5. A hydraulic system according to claim 4,wherein the 2/2 way valve includes a valve spool having a first faceacted upon by the first pressure and having a second face acted upon bythe second pressure and the spring force.
 6. A hydraulic systemaccording to claim 1, wherein the flow regulator includes at least onevalve for switching the pump flows conveyed to thehydraulic-fluid-operated device and to an additional component.
 7. Ahydraulic system according to claim 6, wherein the flow regulatorincludes at least two valves connected in series.